Due to the recent progress in miniaturization processes, the technology in the regime of a gate length of 0.1 μm or less is on the verge of being available for a real application in an ultrahigh speed semiconductor device. In general, while an operating speed of a semiconductor device is improved by miniaturization, accompanied with the reduction of the gate length achieved as a result of the device miniaturization in such an extremely miniaturized semiconductor device, it is imperative to reduce the thickness of a gate insulating film as well in accordance with a scaling rule.
In case the gate length is reduced to 0.1 μm or less, however, it is necessary to set the thickness of the gate insulating film to 1˜2 nm or less when a conventional thermal oxide film is used for the gate insulating film. In such an extremely thin gate insulating film, a tunneling current is increased, which in turn inevitably increases a gate leakage current.
Under these circumstances, there has been a proposal of using a high-K dielectric material (what is called high-K material) having a much larger dielectric constant than that of a thermal oxide film and as a result, having a small SiO2 equivalent thickness despite a large physical film thickness, such as Ta2O5, Al2O3, ZrO2, HfO2, ZrSiO4 or HfSiO4, for the gate insulating film. By using such a high-K dielectric material, it becomes possible to use a gate insulating film of about 10 nm in physical film thickness in ultrahigh speed semiconductor devices having an extremely short gate length of 0.1 μm or less. As a result, the gate leakage current caused by tunneling effect can be suppressed.
From the viewpoint of improving carrier mobility in channel region, it is preferable to interpose an extremely thin base oxide film having a thickness of 1 nm or less, preferably 0.8 nm or less, between the high-K dielectric gate oxide film and the silicon substrate. The base oxide film has to be extremely thin. If it is thick, the effect of using the high-K dielectric film for the gate insulating film would be cancelled out. Further, such an extremely thin base oxide film has to cover the surface of the silicon substrate uniformly, and it should not form defects such as interface states.
Although, in general, a thin gate oxide film has been conventionally formed by performing a rapid thermal oxidation (RTO) processing on a silicon substrate, in order to form a thermal oxide film of a desired thickness of 1 nm or less, it is necessary to lower a processing temperature. However, such a thermal oxide film formed at a low temperature is liable to include defects such as interface states or the like, and inadequate for the base oxide film of the high-K dielectric oxide film.
Under these circumstances, in order to form a base oxide film, the inventor of the present invention has proposed to use a UV-excited oxygen radical (UV-O2 radical) substrate processing apparatus capable of forming a high quality oxide film at a low film forming speed, based on a low radical density in International Patent No. WO03/049173A1.
FIG. 1 provides a schematic configuration of a conventional UV-O2 radical substrate processing apparatus 20 in accordance with the above-described proposal.
Referring to FIG. 1, the substrate processing apparatus 20 includes a substrate supporting table 22 which is provided with a heater (not shown) and is vertically movable between a process position and a substrate loading/unloading position; and a processing container 21 which is partitioned to form a process space 21B along with the substrate supporting table 22, wherein the substrate supporting table 22 is rotated by a driving mechanism 22C. Further, the inner wall surface of the processing container 21 is covered with an inner liner (not shown) made of quartz glass, and thus, metallic contamination of target substrate from an exposed metal surface is suppressed.
The processing container 21 is connected to a substrate transferring unit 27 via a gate valve 27A, and thus, while the substrate supporting table 22 is lowered to a loading/unloading position, a target substrate W is transferred from the substrate transferring unit 27 onto the substrate supporting table 22 via the gate valve 27A, and a processed substrate is transferred from the substrate supporting table 22 to the substrate transferring unit 27.
In the substrate processing apparatus 20 shown in FIG. 1, a gas exhaust port 21A is formed near the gate valve 27A of the processing container 21, and a turbo molecular pump (TMP) 23B is connected to the gas exhaust port 21A via a valve 23A and an APC (Automatic Pressure Controller, not shown). The turbo molecular pump 23B is also connected to a dry pump (DP) 24 via a valve 23C, and by driving the turbo molecular pump 23B and the dry pump 24, it is possible to lower the pressure of the process space 21B to 1.33×10−1˜1.33×10−4 Pa (1×10−3˜1×10−6 Torr).
Further, the gas exhaust port 21A is connected to the pump 24 directly via a valve 24A and another APC (not shown), and thus, by opening the valve 24A, the pressure in the process space 21B is reduced to the level of 1.33 Pa˜1.33 kPa (0.01˜10 Torr) by the pump 24.
The processing container 21 is provided with a processing gas supply nozzle 21D supplying an oxygen gas, at a side opposite to the gas exhaust port 21A across the target substrate W, and the oxygen gas supplied to the processing gas supply nozzle 21D flows along the surface of the target substrate in the process space 21B, and is exhausted through the gas exhaust port 21A.
In order to activate the processing gas thus supplied from the processing gas supply nozzle 21D and to generate oxygen radicals, the substrate processing apparatus 20 shown in FIG. 1 is provided with an ultraviolet light source 25 having a quartz window 25A on the processing container 21 at a position corresponding to a region located between the processing gas supply nozzle 21D and the target substrate W, wherein the ultraviolet light source 25 preferably includes an excimer lamp having a wavelength of 172 nm. Thus, by driving the ultraviolet light source 25, the oxygen gas introduced through the processing gas supply nozzle 21D into the process space 21B is activated to generate oxygen radicals, and the oxygen radicals thus formed flow along the surface of the target substrate W. Accordingly, it becomes possible to form a uniform radical oxide film on the surface of the target substrate W with a thickness of 1 nm or less, particularly with the thickness of about 0.4 nm, which corresponds to the thickness of 2˜3 atomic layers.
In such a process for generating a radical oxide film shown in FIG. 1, by closing the valve 24A and opening the valve 23A, the pressure of the process space 21B is reduced to the range of 1.33×10−1˜1.33×10−4 Pa (1×10−3˜1×10−6 Torr), suitable for performing an oxidation process on a substrate by the oxygen radicals.
Further, the processing container 21 is provided with a remote plasma source 26 at the side thereof opposite to the gas exhaust port 21A with respect to the target substrate W. Thus, it is possible to form nitrogen radicals by supplying a nitrogen gas to the remote plasma source 26 together with an inert gas such as Ar or the like and by activating the nitrogen gas thus supplied with a plasma. The nitrogen radicals thus formed are made to flow along the surface of the target substrate W to thereby nitride the substrate surface as shown in FIG. 2. In the plasma nitriding (RFN) process shown in FIG. 2, by closing the valve 23A and opening the valve 24A, the pressure of the process space 21B is reduced to the range of 1.33 Pa˜1.33 kPa (0.01˜10 Torr). By the plasma nitriding process, it is possible to nitride the extremely thin oxide film of about 0.4 nm in thickness, which has already been formed in the process described with reference to FIG. 1.
However, in such a substrate processing apparatus 20, it has been found that a plasma cannot be easily ignited when the operation is restarted after the processing container 21 has not been operated while the processing container 21 has been left under a vacuum condition for 2˜3 days or longer, or has been open to atmosphere for a maintenance and/or the repair.
To ignite a plasma, the inside of the processing container 21 has to be purged repeatedly over 2˜3 hours. However, such a purge process over a long period of time deteriorates the substrate processing efficiency significantly.